Viral immunoevasins are key molecules employed by viruses to subvert the host immune response during infection. Understanding the molecular basis of their functions is key for explaining how viruses have adapted to specifically infect selected hosts and for the design of new vaccines and other antiviral therapies. Mouse cytomegalovirus (MCMV) has a set of such proteins that specifically interfere with major histocompatibility complex class I (MHC-I) antigen presentation to CD8+ T cells and natural killer (NK) cells. Notwithstanding the large number of genetic and functional studies, the structural biology of immunoevasin specificities and functions is poorly understood, due largely to bottlenecks in co-crystallizing these proteins and their ligands. To bypass this bottleneck I have developed a new approach that combines sparse datasets recorded from nuclear magnetic resonance (NMR) spectroscopy with sophisticated Rosetta modeling methods. This hybrid approach now permits accurate structure determination of protein complexes in the 30kD-50kD range, a size previously unapproachable by NMR techniques alone. I have already applied this approach to elucidate the solution structure of the MCMV m04 immunoevasin. I now propose to extend these methods to determine the structures of m04/MHC-I complexes. These structural investigations can reveal, in atomic detail, the mode of interaction of m04 with MHC-I thereby helping to resolve a longstanding puzzle regarding the function of the MCMV m04 immunoevasin, i.e. why does m04 enhance MHC-I surface expression instead of diminishing it? By analyzing structural aspects and the plasticity of the m04/MHC-I interaction, I aim to identify which of the several unique binding sites on MHC-I for cell surface immune receptors are blocked by the interaction with m04. Therefore, deciphering the mechanism of m04 binding to MHC will elucidate the basis of m04 specificity for particular MHC allotypes and provide insight into the function of the complex with respect to interfering with antigen presentation to T cells and NK cells. Although MCMV only infects mice, it has facilitated many important discoveries for the related human virus (HCMV), which causes a range of congenital infections and diseases in individuals with compromised immune systems (such as HIV patients and transplant recipients). By elucidating a key mechanism by which CMVs bypass immune responses and establish an infectious state, this study will provide insight to HMCV infection and further advance a new technology for structure-based studies of viral immune-interfering proteins.